1. Field of the Invention
This invention relates to integrated circuits, and more particularly, to characteristics of transistors used in integrated circuits.
2. Description of the Related Art
CMOS integrated circuits (IC's) utilize a large number of field effect transistors (FET's). These transistors may be activated or deactivated based on a threshold voltage. More particularly, such transistors may be activated or deactivated based on a relationship between a voltage difference between gate and source terminals (Vgs) and a threshold voltage. For example, an n-type metal oxide semiconductor (NMOS) transistor may be turned on when Vgs meets or exceeds a threshold voltage of the transistor. A p-type metal oxide semiconductor (PMOS) transistor may be turned on when Vgs is less than or equal to the transistor's threshold voltage.
One factor that may affect the threshold voltage for a given transistor is the dopant density in the channel of the transistor. Varying the channel dopant can change the threshold voltage of the transistor. Another factor is its gate oxide thickness. Generally speaking, for otherwise similar transistors, those with a thicker gate oxide may have a higher threshold voltage. The thickness of the gate oxide may have other effects as well.
One such effect of implementing transistors with different channel dopant densities and/or a thick gate oxide (and thus higher threshold voltage) may be the reduction of sub-threshold leakage currents, particularly in low voltage circuits. Accordingly, such transistors may be useful in applications wherein reduced power consumption is desirable.
On the other hand, transistors having a lower threshold voltage may have faster switching times. Thus, transistors having a thinner gate oxide may be useful in applications where speed is a primary consideration.